1. Field of the Invention
The present invention relates to a coplanar waveguide resonator and a coplanar waveguide filter using the same. More specifically, it relates to miniaturization of the same.
2. Description of the Related Art
Recently, a coplanar waveguide filter using one or more coplanar waveguide resonators has been proposed as a filter used in a transceiver device for microwave or millimeter wave communications. A coplanar waveguide resonator has a line conductor (a center conductor) having an electrical length equivalent to a half wavelength or a quarter wavelength and a ground conductor disposed across a predetermined space from the center conductor that are formed on the same surface of a dielectric substrate. Thus, for example, the circuit pattern is formed on only one side of the dielectric substrate, and no via hole is needed to form a short-circuited stub. As a result, the coplanar waveguide resonator has advantages that the manufacturing process is simple and the conductor film can be formed at low cost.
FIG. 27 shows an exemplary conventional coplanar waveguide filter composed of a plurality of half-wavelength coplanar waveguide resonators connected in series with each other (see the non-patent literature 1). A coplanar waveguide filter 900 is formed by forming a ground conductor 903 on the entire surface of a dielectric substrate 905 having the shape of a rectangular plate by vapor deposition or sputtering, and patterning the ground conductor 903 by photolithographic etching, thereby forming half-wavelength coplanar waveguide resonators Q1, Q2, Q3 and Q4, each having a half-wavelength center conductor 901 with two open-circuited ends, that are connected in series with each other in the direction of extension of the half-wavelength center conductors 901. In this example, line conductors 902 formed between adjacent half-wavelength coplanar waveguide resonators connect the ground conductors 903 that are facing to one another in order to suppress an unwanted mode, such as the slotline mode. In FIG. 27, illustration of input/output terminals, which is formed at the opposite ends of the coplanar waveguide resonators (the left and right ends of the coplanar waveguide resonators when the drawing is viewed straight from the front), is omitted. In FIGS. 27 to 29, for the sake of simplicity, stereoscopic representation is partially omitted.
Non-patent literature 1: Jiafeng Zhou, Michael J. Lancaster, “Coplanar Quarter-Wavelength Quasi-Elliptic Filters Without Bond-Wire Bridges”, IEEE Trans. Microwave Theory Tech., vol. 52, No. 4, pp. 1149-1156, April 2004
FIG. 28 shows another exemplary conventional coplanar waveguide filter composed of a plurality of quarter-wavelength coplanar waveguide resonators connected in series with each other (see the patent literature 1 and the non-patent literature 2, for example). A coplanar waveguide filter 910 is composed of quarter-wavelength coplanar waveguide resonators S1, S2, S3 and S4 having a quarter-wavelength center conductor 911, which is short-circuited to a ground conductor 903 at one end and open-circuited at the other end, connected in series with each other in the direction of extension of the quarter-wavelength center conductors 911 in such a manner that adjacent quarter-wavelength coplanar waveguide resonators are disposed in inverted orientations. In other words, two types of parts appear alternately in the coplanar waveguide filter 910, the one of two types being a part in which adjacent two quarter-wavelength coplanar waveguide resonators are disposed with the quarter-wavelength center conductors 911 thereof connected to a line conductor 912 that connects the ground conductors 903 facing to one another, and the other one of two types being a part in which adjacent two quarter-wavelength coplanar waveguide resonators are disposed with the open-circuited ends of the quarter-wavelength center conductors 911 thereof facing each other. Furthermore, to improve the coupling strength of a capacitive coupling part C at which the open-circuited ends of the quarter-wavelength center conductors 911 face each other, changing the shapes of the open-circuited ends at the capacitive coupling part C is permitted in such a manner that the area of the parts of the open-circuited ends facing each other increases. Patent literature 1: Japanese Patent Application Laid-Open No. H11-220304 Non-patent literature 2: H. Suzuki, Z. Ma, Y. Kobayashi, K. Satoh, S. Narahashi and T. Nojima, “A low-loss 5 GHz bandpass filter using HTS quarter-wavelength coplanar waveguide resonators”, IEICE Trans. Electron., vol. E-85-C, No. 3, pp. 714-719, March 2002
As is apparent from comparison between the examples described above, for the same resonance frequency, the total length of the coplanar waveguide filter composed of a plurality of quarter-wavelength coplanar waveguide resonators connected in series with each other is shorter than that of the coplanar waveguide filter composed of a plurality of half-wavelength coplanar waveguide resonators connected in series with each other, because the quarter-wavelength center conductors of the quarter-wavelength coplanar waveguide resonators have an electrical length equivalent to a quarter wavelength shorter than that of a half wavelength.
Furthermore, there is a known coplanar waveguide filter structure shown in FIG. 29 in which the quarter-wavelength center conductors of the quarter-wavelength coplanar waveguide resonators have a stepped impedance structure to reduce the total length of the coplanar waveguide filter (see the non-patent literature 1).
The total length of the coplanar waveguide filter composed of a plurality of coplanar waveguide resonators connected in series with each other in the direction of the connection (referred to simply as the total length of the coplanar waveguide filter, hereinafter) largely depends on the total length of each of the coplanar waveguide resonators forming the coplanar waveguide filter in the direction of the connection (referred to simply as the total length of the coplanar waveguide resonator, hereinafter). If the total length of the coplanar waveguide resonator is reduced, the total length of the coplanar waveguide filter composed of the coplanar waveguide resonators is also reduced.
Although the quarter-wavelength coplanar waveguide resonator has a shorter total length than the half-wavelength coplanar waveguide resonator, the center conductor has to have a physical length corresponding to an electrical length equivalent to a quarter wavelength at a desired resonance frequency, and it is necessary to contemplate further reducing the total length of the quarter-wavelength coplanar waveguide resonator.
If the stepped impedance structure is used in the quarter-wavelength coplanar waveguide resonator, the total length of the coplanar waveguide resonator can be further reduced. However, the area of the center conductor is increased to increase the capacitance at the part at which the electrical field is concentrated, and therefore, it is difficult to reduce the footprint of the quarter-wavelength coplanar waveguide resonator on the dielectric substrate, while the total length of the coplanar waveguide resonator can be reduced.
Alternatively, the total length of the coplanar waveguide resonator can be further reduced if the center conductor is formed in a meander or spiral shape. However, the quarter-wavelength coplanar waveguide resonator requires an area on which the center conductor having a physical length corresponding to an electrical length equivalent to a quarter wavelength is disposed, and therefore, it is difficult to reduce the footprint of the quarter-wavelength coplanar waveguide resonator on the dielectric substrate.
As described above, even if the total length of the coplanar waveguide resonator can be reduced, the coplanar waveguide resonator cannot be sufficiently miniaturized.